The marines only take up 5% of the budget on paper. The vast majority of their procurement comes out of the navy budget!...just because they only take up a small part of the budget doesn't negate that their air power capabilities are the least justifiable.
The UK would either get Rafale or the F18.
I would expect a mix of both not hitting the peak full rate production and ending the programme early. The problem is that there is going to be less money around in 15 years time than had been forecast 5 years ago; now that we are working on a new trend budget line (producing a $1 trillion dollar differential over 10 years), the only way of realistically reaching the total aspirational numbers are through massive increases in the defense budget through the early 2020s, which is extremely unlikely in any situation, but even more so given the federal government's $17 trillion worth of debt.